Department of Physiology, School of Medicine, Karl-Franzens-University Graz, Austria
Purpose: The mass density of antecubital venous blood was measured continuously for 80 min / session with 0.1 g/l precision at a flow rate of 1.5 ml/min in six male subjects. Each person participated in two different sessions with the same protocol. To induce transvascular fluid shifts, the subjects changed from sitting to standing and from standing to supine positions.
Results: There was transient blood density shifts immediately after postural changes, followed by an asymptotic approach to a new steady-state blood density level. Additional deviations from a simple time course were regularly observed. Blood density increased by 3.5 ± 1.4 (SD) g/l when standing after sitting and decreased by 5.0 ± 1.2 g/l while supine after standing. The corresponding half time of the blood density increase was 5.6 ± 1.4 min (standing after sitting) and 6.9 + 3.1 min (supine after standing) of the blood density decrease. Erythrocyte density was calculated and did not change with body position. Whole-body blood density was calculated from plasma density, hematocrit, and erythrocyte density, assuming an F-cell ratio of 0.91. Volume shifts were computed from the density data; the subject's blood volume decreased by 6.2 ± 1.2% from sitting to standing and increased by 8.5 ± 2.1% from standing to supine. Additional discrete plasma density and hematocrit measurements gave linear relations (P<0.001) between all possible combinations of blood density, plasma density, and hematocrit.
Conclusion: On-line blood densitometry can be used for continuous monitoring of blood volume changes in humans and reveal transient filtration phenomena that are not amenable to discontinuous measurements.
>> Continuous recording of blood density in 6 subjects. Dotted interconnections or short breaks indicate when the measuring systemn was rinsed with heparinized saline solution. Measurement temperature: 37.0șC.